1. Field of the Invention
The present invention relates to a pressure sensor, and more specifically, to a semiconductor pressure sensor capable of avoiding generating leakage currents for meeting market requirements.
2. Description of the Prior Art
Air pressure and hydraulic measurements are important in industrial control. Currently, a pressure sensor in common use includes a piezoresistive pressure sensor, a piezoelectric pressure sensor, a capacitive pressure sensor, a potentiometer pressure sensor, an inductive-bridge pressure sensor, a strain gauge pressure sensor, and a semiconductor pressure sensor.
Additionally, because sizes of various pressure sensors are reduced gradually, a micromachining technology is developed to manufacture various microsensors and microactuators that are integrated with micro electronic circuits to form a microsystem, which is generally called a micro electro-mechanical system (MEMS). The MEMS has an extremely small size and can be manufactured by utilizing batch production for reducing a production cost. In addition, the MEMS and a signal processing circuit can be simultaneously formed on a silicon wafer for forming a monolithic device, which can reduce a distance between a pressure sensor and the signal processing circuit and that is quite important for pressure sensors. As the pressure sensor outputs a signal, the signal is firstly amplified by the signal processing circuit for preventing the signal from being disturbed by an ambient electromagnetic field, and the signal can be analog-to-digital (A/D) converted by the signal processing circuit and be transmitted to a central processing unit. Therefore, as the distance between the pressure sensor and the signal processing circuit is reduced, signal reliability can be greatly improved, and interconnecting lines and loads of central control systems can be effectively decreased. As a result, the pressure sensor that is manufactured by use of MEMS is developed rapidly due to its advantages of good detection sensitivity and a low production cost. Additionally, among the above-mentioned kinds of pressure sensors, the piezoresistivepressure sensor has advantages of a high output voltage and high detection sensitivity, while the piezoelectricpressure sensor has advantages of high detection sensitivity, a low electromagnetic interruption, low power dissipation, high energy density, a fast response, and low sensitivity to an ambient environment. Accordingly, the piezoresistive pressure sensor and the piezoelectric pressure sensor are usually applied on microsensors and microactuators in the MEMS field.
Please refer to FIG. 1. FIG. 1 is a sectional view of a conventional piezoresistive semiconductor pressure sensor 10. As shown in FIG. 1, the piezoresistive semiconductor pressure sensor 10 comprises an etched semiconductor substrate 12, such as a single-crystal silicon substrate or a silicon-on-insulator (SOI) substrate. The etched semiconductor substrate 12 includes a diaphragm 14, a base 16 for fixing two ends of the diaphragm 14 and forming a cavity 18 under the diaphragm 14, and a piezoresistor 20 positioned in the diaphragm 14 and used as a sensing device of the piezoresistive semiconductor pressure sensor 10. Additionally, the piezoresistive semiconductor pressure sensor 10 further comprises a control circuit 22 positioned in the etched semiconductor substrate 12 and electrically connected to the diaphragm 14 and the piezoresistor 20. The control circuit 22 mainly includes a complementary metal-oxide semiconductor (CMOS), an amplifying circuit, or a logic circuit, and the control circuit 22 functions to receive, process, and transmit signals output from the piezoresistor 20.
For forming the etched semiconductor substrate 12, an anisotropic etching process is usually performed to etch a reverse side of a semiconductor substrate through use of an etchant, such as potassium hydroxide (KOH), so as to form the diaphragm 14 and the base 16, whose areas and thickness conform to process requirements. Additionally, a portion of the diaphragm 14 is implanted with boron (B) through use of a thermal diffusing method or an ion implantation process for forming the piezoresistor 20. Because the diaphragm 14 comprises single-crystal silicon, a p-n junction can be formed after the diaphragm 14 is implanted with boron (B), and the p-n junction forms the piezoresistor 20 that is used to measure a variation of pressure.
When a pressure to be measured is exerted on the diaphragm 14, or a pressure difference is generated between the inside and the outside of the diaphragm 14, a central portion of the diaphragm 14 will be deformed, thereby causing the piezoresistor 20 to be deformed. Therefore, a resistance of the piezoresistor 20 will be altered due to surface expansion or surface contraction of the piezoresistor 20. Thereafter, the control circuit 22 performs a signal process, such as signal amplification or temperature compensation, and the control circuit 22 converts a resistance variation of the piezoresistor 20 into a differential signal. Finally, the control circuit 22 outputs the differential signal, which corresponds to the pressure to be measured.
Alternatively, if the piezoresistor 20 is formed with a piezoelectric thin film, such as ZnO, the semiconductor pressure sensor 10 becomes a piezoelectric semiconductor pressure sensor. The piezoelectric semiconductor pressure sensor is driven according to the piezoelectric effect. As a pressure is applied on the piezoelectric thin film, electric charges are generated at two ends of the piezoelectric thin film and the amount of the electric charges are directly proportional to the pressure. Accordingly, the pressure can be obtained through measuring a variation of the electric charges.
The conventional piezoresistive semiconductor pressure sensor and the conventional piezoelectric semiconductor pressure sensor are both composed of single-crystal silicon, so that the conventional semiconductor pressure sensor 10 has good detection sensitivity. However, a cost of silicon wafer is so high that it costs a lot to form the conventional semiconductor pressure sensor 10. As a result, it is an important issue to manufacture a semiconductor pressure sensor with a low production cost and a high quality. Additionally, since the piezoresistor is formed through use of a thermal diffusing method or an ion implantation process in the prior art, the p-n junction is therefore formed between the piezoresistor and the diaphragm of single-crystal silicon. Nevertheless, leakage currents are usually generated near the p-n junction, thus disturbing an operation of the sensor.